1. Field of the Invention
This invention relates to a mercury free arc tube for a discharge-lamp device provided with a sealed glass chamber in which at least a metallic halide for main light emission is sealed as well as a rare gas by pinch-sealing both end openings of a glass tube and electrode bars are provided so as to oppose to each other. This invention particularly relates to a mercury free arc tube for a discharge-lamp device provided with electrode bars each having such a concentric stepped shape in which a cross sectional area of a tip side region projecting into the sealed glass chamber is larger than that of a base side region sealed on a pinch-sealed portion.
2. Description of the Background Art
FIG. 9 illustrates a related art discharge lamp device. A front end of an arc tube 5 made of quartz glass is supported by a single lead support 2 which projects forward of an insulating base 1. A rear end of the arc tube 5 is supported by a concave portion 1a of the insulating base 1. An area adjacent to the rear end of the arc tube 5 is held by a metallic supporting member 4 secured to a front face of the insulating base 1. Lead wire 8 on the front end side led out from the arc tube 5 is fixed to the lead support 2 by welding. On the other hand, the lead wire 8 on the rear end side passes through a bottom wall 1b on which the concave portion 1a of the base 1 is formed and fixed to a terminal 3 formed on the bottom wall 1b by welding. Symbol G denotes a cylindrical glass globe for cutting off a component of ultraviolet rays, which have a wavelength that is harmful to the human body and which is emitted from the arc tube 5. The globe G is integral with the arc tube 5.
The arc tube 5 has a structure in which between a pair of front and rear pinch-sealed portions 5b, 5b, a sealed glass chamber 5a is formed in which electrode bars 6, 6 are opposite to each other and a light emitting material (halide of Na or Sc and Hg) is sealed with rare gas. Within each of the pinch-sealed portions 5b, a molybdenum foil 7 is deposited for connecting the electrode bars 6 projecting into the sealed glass chamber 5a and the lead wire 8 led out from the pinch-sealed portion 5b, thereby assuring hermeticity of the pinch-sealed portions 5b. 
Specifically, the electrode bar 6 is preferably made of tungsten having excellent heat resistance and high endurance. However, tungsten has a linear expansion coefficient which is greatly different from that of the quartz glass constituting the arc tube and poor familiarity with the quartz glass, thus giving inferior hermeticity. In view of this, by connecting the molybdenum foil 7 having excellent expandability and flexibility and better familiarity with the quartz glass to the electrode bar 6 of tungsten and sealing the molybdenum foil 7 with the pinch-sealed portion 5b, the hermeticity of the pinch-sealed portion 5b is assured.
However, a large temperature difference in the pinch-sealed portion 5b occurs between “on” and “off” of the arc tube. Between the electrode bar and quartz glass which are largely different in their linear expansion coefficient, thermal stress is generated during the “on” state of the arc tube. Particularly, since the arc tube in recent years is designed so that it can be instantaneously turned on, the rate of temperature rise is large and so the thermal stress is abruptly generated. If this status is repeated by on/off of the arc tube, in the pinch-sealed portion (quartz glass layer) 5b which seals the electrode bar 6, cracks (hereinafter referred to as longitudinal cracks) extending radially from the electrode bar 6 are generated so that the sealed substance will leak. This leads to a problem of poor lighting or reduction of the life of the arc tube.
In order to cope with this problem, Japanese Patent Unexamined Patent Publication JP-A-2001-15067 has been proposed on the basis of the idea that the longitudinal crack is more difficult to be generated in the quartz glass layer of the pinch-sealed portion 5b in the case where residual compressive distortion remains over a predetermined region, because the thermal stress generated in the quartz glass layer of the pinch-sealed portion is dispersed with the rise of temperature due to lighting of the arc tube, thereby extending the life of the arc tube.
More specifically, JP-A-2001-15067, as seen from FIG. 10, proposes a structure in which on a face in intimate contact with the electrode bar 6 of the quartz glass layer of the pinch-sealed portion 5b, a residual compressive distortion layer 9 is formed over a predetermined wide range. Also, between the residual compressive distortion layer 9 and its encircling glass layer, a bead crack 9a is formed. Note that the bead crack 9a is a crack extending circumferentially and axially so as to surround the residual compressive distortion layer 9. In this structure, when the arc tube is turned on, the thermal stress generated in the interface between the electrode bar 6 and the quartz glass layer is absorbed and dispersed by the residual compressive distortion layer 9 and the bead crack 9a and conducted toward the quartz glass layer. Thus, the longitudinal crack leading to leakage of the sealed substance is not generated in the quartz glass layer of the pinch-sealed portion 5b. 
Mercury (Hg) sealed in the sealed glass chamber 5a is a very useful substance to keep a predetermined tube voltage and to reduce the quantity of collisions of electrons with the electrode to thereby alleviate damage of the electrode. However, since Hg is harmful to the environment, in recent years, development of a “mercury-free arc tube” in which Hg is not contained has been advanced.
In the case of a “mercury-free” arc tube, the tube voltage is lowered so that the tube electric power necessary for discharging cannot be obtained. So, in order to increase the tube electric power, it is necessary to increase the current (tube current) to be supplied to the arc tube. The load of the electrode is correspondingly increased so that the electrode is injured (consumed or blacks). This leads to a problem of reduction in the light emission efficiency and extinction of arc. This problem can be solved by increasing the diameter of the electrode bar 6. However, the following situation may occur. Namely, if the electrode bar 6 is too thick, a difference in the quantity of heat-shrinkage between the electrode bar and the quartz glass layer becomes great, so that they will be separated from each other at the interface therebetween. As a result, a residual compressive distortion layer 9 and bead crack 9a around the electrode bar 6 in the quartz glass layer of the pinch-sealed portion 5b having an optimum size capable of absorbing/alleviating the thermal stress generated when the arc tube is turned on cannot be formed. Accordingly, by “on/off” of the arc tube, the longitudinal crack leading to leakage of the sealed substance will be generated in the pinch-sealed portion 5b. 
Mercury-free arc tubes disclosed in Japanese Patent Unexamined Publications JP-A-2005-142072 and JP-A-2005-183164 provide a solution of the contradictory problem of injury of the electrode and generation of the longitudinal crack. In the JP-A-2005-142072 and JP-A-2005-183164, by adopting a stepped electrode bar in which, as shown in FIG. 11, the outer diameter of the tip side region 6a of the electrode bar projected into the sealed glass chamber is made larger than that of the base side region 6b of the electrode bar deposited on the pinch-sealed portion. That is, the outer diameter of the base side region 6b of the electrode bar deposited on the pinch-sealed portion is made smaller than that of the tip side region 6a of the electrode bar projected into the sealed glass chamber.
However, the following matter has been confirmed. In the mercury-free arc tube in which the pressure of rare gas within the sealed glass chamber is required high and the turn-on power is required high, particularly when the discharge lamp is actuated, an electric current and heat current, which flows from the electrode tip side region of the electrode bar having a larger diameter toward the electrode base side region having a smaller diameter deposited on the pinch-sealed portion, will be abruptly generated. So, the adoption of only the stepped electrode bar as in JP-A-2005-142072 and JP-A-2005-183164 is insufficient to surely suppress occurrence of flicker (flicker of arc) leading to the reduction of the life of the arc tube and the longitudinal crack or “foil float” in the pinch-sealed portion. Note that the foil float means a case where a gap is generated between the molybdenum foil and the glass layer.